For photosensitive layers of the electrophotographic photoreceptor, inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide, etc, have been generally used. However, they have defects. Selenium and cadmium sulfide are poisonous and they must be recovered after use. Selenium is inferior in heat resistance because it is crystallized by heat. Zinc oxide is inferior in moisture resistance and lacks printing durability. Therefore, many attempts are being made in development of new photoreceptors.
Recently, use of organic photoconductive materials for photosensitive layers of the electrophotographic photoreceptor has been studied and some of them now has come into practical use. Organic photoconductive materials have advantages in comparison with inorganic ones in that they are of light-weight, easily formable into a film and a photoreceptor and some of them can give transparent photoreceptor.
Although organic photoconductive materials have many such advantages, they are not widely used because they are inferior to inorganic ones in sensitivity and durability.
In the mean time, it was found that the so-called layered type photoreceptor comprising a charge generating layer and a charge transporting layer provided on an elctroconductive support is advantageous in enhancement of sensitivity and thus the layered type photoreceptor is the main stream of development, in which organic photosensitive materials have now come into use.
However, layered type photoreceptors now in practical use are inferior to inorganic photoreceptors in durability in particular. One of the elements which decide durability is physical properties. That is, organic photoreceptors are liable to suffer abrasion and scratching by development with toner, friction with paper, friction with a cleaning member (although load is different according to type of apparatuses), etc. Therefore, organic photoreceptors practically have only limited printing durability. In the layered type photoreceptor, it is generally the charge transporting layer that bears such load. A charge transporting layer usually comprises a binder resin and a charge transporting material and the strength of the layer depends on the binder. As the amount of the doped charge transporting material is considerably large, the layer is not provided with sufficient mechanical strength.
For charge transporting layers, polymers or copolymers of vinyl compounds such as styrene, vinyl acetate, vinyl chloride, acryl esters, methacryl esters, butadiene, etc. and thermoplastic and thermosetting resins such as polyvinylacetal, polycarbonate, polyester, polysulfone, poly(phenylene oxide), polyurethane, cellulose esters, cellulose ethers, phenoxy resins, silicon resins, epoxy resins, etc. are used as binders. Among many binder resins, polycarbonate resins have comparatively excellent properties and many kinds of polycarbonate resins have been developed and some are practically in use.
However, even these polycarbonate resins are still insufficient in abrasion resistance, scratch resistance, etc., especially in abrasion resistance when polycarbonate resins are used for the electrophotography and thus development of binder resins which are provided with excellent abrasion resistance is strongly desired.
The present invention was made in order to overcome the above-described problem and is intended to provide a binder resin which enables manufacturing of highly abrasion-resistant photoreceptors having excellent durability.
In this respect, we conducted an intensive study in search of binder resins which can provide durable electrophotographic photoreceptors and found that a particular polycarbonate resin having a weight average molecular weight of not less than 200,000.
In the mean time, photoreceptors are conventionally prepared by dip coating. For instance, a cylindrical substrate is dipped in a batch of a coating liquid. In this method, run (local flow-down) of the liquid on the coated surface is apt to occur. Also local nonuniformity in concentration of the coating liquid is caused in the batch. Thus it is difficult to form a coating layer of an even thickness. Generally, binder resins of higher molecular weights have higher viscosities and thus are liable to promote occurrence of the above-mentioned run and concentration nonuniformity. Generally, as the molecular weight of the binder resin increases, the viscosity thereof increases, and thus the coating liquid which contains the polycarbonate resin in accordance with this invention becomes viscous. Such a viscous coating liquid easily develop concentration nonuniformity and bubbling and that the once caused concentration uniformity and bubbles will not be remedied. The means to solve this problem is to reduce the solid content of the coating liquid. However, if the coating is effected with a coating liquid with a low solid concentration, the above-mentioned run is easier to occur when a cylinder is coated by dip coating since the solvent content in the wet coating is large. Therefore, it is not easy to form a photosensitive layer of even thickness from the coating liquid containing the polycarbonate resin having a weight average molecular weight not less than 200,000.
Thus we conducted an intensive study concerning method for forming photosensitive layer of uniform thickness from the above described polycarbonate resin of the present invention and found that spiral coating or spray coating is effective.